The oldest rocks in Wales

The oldest objects in the Museum's collections are 4,500 million year-old meteorites from space. However, the oldest objects from Wales are rock specimens from the Old Radnor district, Powys, which formed about 700 million years ago, when Wales as we know it today had yet to take shape.

Molten rock from Avalonia

These rocks form the three small distinctive conical hills of Hanter, Stanner and Worsel Wood, and are composed of gabbro, dolerite, and granite — igneous rocks that were once molten. When these formed, around 700 million years ago, Wales lay deep in the southern hemisphere. Along with England and parts of Newfoundland, New Brunswick and New England it formed part of the mico-continent Avalonia, which lay on the margin of the vast continent of Gondwana.

At this time Scotland was close to the equator, attached to the North American continent of Laurentia, and it was hundreds of millions of years later that it separated and merged with southern Britain.

Very old rocks are also found in Anglesey and the Lleyn Peninsula, and similar rocks probably underlie the whole of Wales. Typically we only see these ancient rocks where they have been brought to the surface by upward movement along major fault zones.

How do we know that these are the oldest rocks in Wales?

These rocks contain no fossils. So how do we know that those from the Old Radnor district are the oldest, and what age they are? The answer lies in the minerals from which a rock is made, and the chemical elements that these minerals contain.

Radioactive decay

Rocks are aggregates of minerals, which in turn are composed of chemical elements. Many elements occur in different forms, or isotopes, and some of these are naturally unstable and change spontaneously into a different element by the process of radioactive decay. This process provides a natural clock that measures the time that has elapsed since the minerals formed. The clock starts to 'tick' as soon as the mineral crystallises.

By using high precision equipment — a mass spectrometer — the proportion of radioactive decay products can be measured. As these accumulate at a constant rate, we can calculate the time necessary for them to have formed. This is typically many millions of years.

The Stanner-Hanter rocks have been dated using two different sets of isotopes. The decay of rubidium into strontium gave an age of 702 ± 8 million years whereas the uranium-lead system provided a slightly more precise age of 710 ± 1.5 million years.

The oldest rocks in Wales, although 700 million years old, are significantly younger than the oldest rocks found in the British Isles and Europe. These are metamorphic rocks from the Isle of Lewis, Scotland, dated at 3,300 million years old.

The oldest rocks in the world

The oldest dated rocks known in the world are 3,962 million years old, and come from Acasta, north-west Canada. Even these were formed from older, as yet undated, rocks.

Samples of the oldest rocks from Wales, the British Isles and the world can be seen in the Evolution of Wales exhibition at National Museum Cardiff.